A power supply controller is conventionally provided, in which a high-power semiconductor switching element such as a power MOSFET is disposed on a power supply line connected between a power source and a load, and which is configured to control the power supply to the load by switching the semiconductor switching element between ON and OFF. As shown in JP-A-2001-217696, some of such power supply controllers have a self-protective function. The self-protective function turns off the semiconductor switching element in response to an occurrence of overcurrent by controlling the potential of the control terminal of the semiconductor switching element so as to shut off the current supply, and thereby protects the semiconductor switching element. Specifically, a current detecting resistor is serially connected to a current conducting terminal (e.g., the source or drain in the case of a MOSFET) of the semiconductor switching element, for example. The voltage drop on the resistor is detected, and the occurrence of overcurrent is determined if the voltage drop is larger than a predetermined level.
However, even in the case that the power supply controller having a self-protective function described above is used, an external circuit (e.g., a wiring member (such as a wire)) may produce heat due to the power supply resulting in burnout, if the self-protective function performs forcing ON-OFF operations in succession so that the semiconductor switching element is intermittently turned to a conductive state or a shutoff state. For this reason, an additional fuse element is conventionally provided on the power supply line in consideration of the load resistance of the external circuit or the like.
Note that a wiring member typically has a construction in which a plurality of stranded wires are bundled together and covered with a coating, for example. The fuse value of the fuse element is determined in consideration of heat produced when an overcurrent passes through all the stranded wires. However, the construction utilizing a fuse element may fail to protect the wiring member. That is, a chattering short may occur, if a fraction of the stranded wires are exposed to the outside of the coating due to partial damage of the coating of the wiring member and thereby the fraction of the stranded wires intermittently forms a short circuit with a surrounding conductive member (e.g., a vehicle body) due to a vibration of the vehicle on which the power supply controller is installed, for example. When a chattering short of some duration occurs in some periods, the amount of an overcurrent passing into the fuse element will not reach an extent of fusing. However, an overcurrent may pass through the short-circuiting fraction of the stranded wires, and thereby heat may be locally produced so that the coating on the periphery thereof smokes. To cope with this, the fuse value of the fuse element can be determined based on the amount of an overcurrent passing through the above fraction of the stranded wires. However, this solution is not desirable, because the fuse element may fuse due to an inrush current, i.e., due to a high current occurring when power supply to the load starts, for example.
In some cases, an FET having an overheat protective function is used as a semiconductor switching element. The FET having an overheat protective function includes a temperature sensor for detecting the temperature of the FET, and turns the FET to a shutoff state if the temperature rises to a predetermined temperature due to an overcurrent passing between the drain and source when short-circuiting occurs in an external circuit, for example. In the case that such an FET having an overheat protective function is used, the above predetermined temperature is also based on an overcurrent passing through all the stranded wired of the wiring member, and therefore the problem arises that a wiring member cannot be protected in some cases similarly to the above-described construction using a fuse element.
Thus, there is a need in the art to protect an external circuit even when an intermittent overcurrent such as a chattering short occurs.